The use of optical fibers in telecommunication networks is gaining favor as a method for increasing the capacity, and providing longer communication links and system economy for the network. Since the bandwidth, or the information carrying capacity of optical fiber, is about 200 nm (25 THz) at 1550 nm, wavelength-division multiplexing can fit 256, 0.8 nm channels within this bandwidth. To take advantage of this fiber capacity, there is a need for an inexpensive, high speed, wide range tunable optical filter.
Several optical tunable filters are now available in the commercial market or are under extensive research. Several are based on Fabry-Perot (FP) resonance cavities. A Fabry-Perot filter is simply a cavity enclosed between two mirrors. The wavelength selected (the resonant peak) depends on the optical path length between the two mirrors. The filter can be tuned either by changing the cavity length mechanically or by changing the refractive index of the material inside the cavity. Mechanical modulation of the cavity length generally is achieved by applying a voltage to the piezoelectric material, which typically has a milliseconds response time. Refractive index modulation of the material within the cavity also has been achieved using nematic liquid crystals. However, the response time is still limited to milliseconds.
In our previous high speed tunable filter work (Johnson et al., U.S. patent application Ser. No. 08/056,415, filed May 3, 1993), homeotropically or tilted layer aligned SmA* liquid crystalline material with lateral electrodes is used within a Fabry-Perot cavity. In the homeotropic alignment the layers are parallel to the cell walls and in the tilted alignment they are at an angle to the walls. Application of an electric field parallel to the cell walls by lateral electrodes rotates the molecular directors in a plane containing the polarization vector of the incident optical field, thereby tuning the material birefringence and modulating the phase of the incident light. Within a Fabry-Perot cavity the phase modulation produces wavelength tuning.
Surface-stabilized planar aligned smectic liquid crystals are increasingly finding application, offering microsecond switching times and either analog (SmA*) or discrete (SmC* and antiferroelectric) switching. Due to their planar alignment with respect to glass restraining substrates smectic layers perpendicular to cell walls, the molecules rotate in a plane perpendicular to the direction of propagation of light through the device and do not provide variable phase modulation. Rotation of the molecules (SmC* or SmA*) in the plane of the substrate can provide a modulation of the intensity of an incident optical field when viewed through crossed polarizers, but can not provide variable retardation. However, in a Fabry-Perot resonator, it is necessary to modulate the phase of the incident optical field to tune the wavelength of maximum transmission of the FP device.
Chiral smectic C* liquid crystals have microsecond response due to their first order coupling between their macroscopic polarization and an applied electric field. In the planar (bookshelf) alignment, as positive and negative electric fields are applied to the cells, the molecules switch between a first and a second stable state, both of which have molecular directors in a plane parallel to the cell walls. It is widely accepted that chiral smectic C* liquid crystals are binary and are incapable of providing analog modulation.
In surface stabilized ferroelectric liquid crystals (SSFLC), such as the SmC* phase, it has been found that the smectic layers are not necessarily perpendicular to the cell walls but may have tilted layers or may contain chevron defects. In the chevron structure the layers lean at an angle to the cell wall and have an interface at which the leaning direction is reversed. Considerable effort has been spent finding materials which are not prone to chevron defects and finding alignment techniques which suppress the chevron structure. Typically the cell thickness is set below 2 .mu.m so that the surface stabilization persists throughout the liquid crystal structure.